There is a conventionally known method of providing a display device with an input function using a touch panel, pen input, a scanner, or the like by providing multiple photosensors in a display panel. In order for this method to be applied to mobile devices used in various light environments, the influence of the light environment needs to be eliminated. In view of this, there is also known to be a method in which the component of a signal detected by a photosensor that is dependent on the light environment is removed from the signal so as to obtain the signal that was originally intended to be input.
Japanese Patent No. 4072732 discloses an input/output device in which a photoreception element is provided in correspondence with each display element, a backlight is blinked one time in a frame period, and resetting and readout are performed with respect to the photoreception elements in a line-sequential manner such that the light amount in the backlight lit period and the light amount in the backlight extinguished period in the frame period are acquired from all of the photoreception elements.
FIG. 24 is a diagram showing the timing of lighting and extinguishing of the backlight disclosed in Japanese Patent No. 4072732 and the timing of resetting and readout with respect to the photoreception elements. As shown in FIG. 24, the backlight is lit in the former half of a frame period and extinguished in the latter half. In the backlight lit period, resetting of the photoreception elements is performed in a line-sequential manner (solid line arrow), and thereafter readout from the photoreception elements is performed in a line-sequential manner (broken line arrow). In the backlight extinguished period as well, resetting and readout with respect to the photoreception elements are performed in a similar manner.
Japanese Patent No. 3521187 discloses a solid-state imaging device that includes the unit photoreception portion shown in FIG. 25. The unit photoreception portion shown in FIG. 25 includes one photoelectric conversion portion PD and two charge storage portions C1 and C2. When both external light and light from a light emitting means that has been reflected by an object are received, a first sample gate SG1 is turned on, and charge generated in the photoelectric conversion portion PD is stored in the first charge storage portion C1. When only external light is received, a second sample gate SG2 is turned on, and charge generated in the photoelectric conversion portion PD is stored in the second charge storage portion C2. The amount of light from the light emitting means that was reflected by the object can be obtained by obtaining the difference between the amounts of charge stored in the charge storage portions C1 and C2.